XcbDpmsHelper: check DPMS state before calling `xcb_dpms_force_level()`
According to Xorg docs[1], BadMatch error returned by DPMSForceLevel() means DPMS is disabled, so check DPMS state before calling it, like what mutter already does in meta_monitor_manager_xrandr_read_current_state().
[1] https://www.x.org/releases/X11R7.7/doc/libXext/dpmslib.html#DPMSForceLevel
(cherry picked from commit https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3271)